1. Field of the Invention
The present invention relates to electromagnetic direction finding, and more particularly, to using a ring interferometer to determine the azimuth and elevation angles of an emitter.
2. Related Art
Techniques of interferometry are commonly used in electromagnetic direction finding systems. For instance, radar interferometry may be used to determine the angle of arrival of a signal. An interferometer accomplishes this task by comparing the phase of the signal received at separate antennas or separate points on the same antenna By comparing the difference in phase of the received signals, the interferometer can determine the angle of arrival of the signal. The angle of arrival of a signal is generally described in terms of the azimuth and elevation angles. The azimuth angle is the angle between a horizontal reference direction and the direction of interest. The elevation angle is the angle that the signal makes with the horizontal plane.
A number of different antenna element configurations are used in interferometers. Linear interferometer configurations consist of antenna elements placed in a straight line. Planar interferometer configurations consist of a number of antenna elements in arbitrary planar arrangements. Linear interferometers and planar interferometers have been used in systems with broad frequency coverage. Methods for determining linear interferometer configurations are described by Robert L. Goodwin in "Ambiguity-Resistant Three- and Four-Channel Interferometers" (NRL Report 8005; Sep. 9, 1976). Methods for determining planar interferometer configurations are described by Neil J. Malloy in "Analysis and Synthesis of General Planar Interferometer Arrays" (IEEE ICASSP Proceedings; Apr. 14-16, 1983).
The methods described by Goodwin and Malloy depend upon using special integer relationships among the spacings of the antenna elements. Special integer relationships among the antenna element spacings are necessary for acceptable ambiguity resistance. Applications arise where it becomes difficult, if not impossible, to provide for these special integer relationships among antenna element spacings. For example, this difficulty arises when using pairs of antenna elements of like polarization that are mounted diametrically opposite each other in forward-boresighted ring arrays for tactical aircraft and missiles. The antenna elements are distributed around the missile body or around the structure of the aircraft behind the nose radome. This non-linear and non-planar antenna element configuration makes it extremely difficult to provide for the special integer relationships among antenna element spacings. What is needed is an interferometry system that does not require special integer relationships among antenna element spacings.